Hexaarylbiimidazole hydroxyphthalein compositions

ABSTRACT

VISIBLE-LIGHT ACTIVATED PHOTOTROPIC COMPOSITIONS OF A HEXAARYLBINIDAZOLE THAT NORMALLY REQUIRES ULTRAVIOLET LIGHT OF ACTIVATION AND A HYDROXYPHTHALEIN DYE THAT IS A VISIBLE-LIGHT ABSORBIND ENERGY-TRANSFER AGENT, AND OPTIONALLY, AN OXIDIZABLE LEUCO FORM OF DYE, PREFERABLY AN AMINOTRIARYLMETHANE. A METHOD FOR IRRADIATING THE COMPOSTION WITH VISIBLE LIGHT IN THE 400-600 MU RANGE.

United States Patent O 3,563,750 HEXAARYLBIIMIDAZOLE HYDROXYPHTHALEIN COMPOSITIONS Peter Walker, Red Bank, N.J., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Filed July 20, 1967, Ser. No. 654,720

Int. Cl. G03c 1/72 US. CI. 96-90 18 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION (1) Field of the invention This invention concerns novel visible-light sensitive phototropic compositions. More specifically, the invention is directed to visible-light activated phototropic compositions comprising a selected hexaarylbiimidazole and a hydroxyphthalein dye.

(2) Description of the prior art Hexaarylbiimidazoles dissociate upon exposure to ultraviolet light to form stable colored triarylimidazolyl radicals as described in British Pat. 997,396, published July 7, 1965. Such dissociation is useful in hexaarylbiimidazole/leuco dye compositions, for the imidazolyl radical, formed as described above, oxidizes the leuco form of the dye to the colored form. Thus, colored images are obtained making the compositions useful in imaging applications, as described in British Pat. 1,047,569, published Nov. 9, 1966. That ultraviolet light is required to activate the biimidazoles is a disadvantage, for in many applications it is desired to avoid such high-energy light because of its adverse effect on other constituents, or it is desired to use instead less costly lower energy visible light. Moreover, in some imaging applications, it is desired to cover the biimdazole-leuco dye composition with a transparent film. Some film materials, such as Mylar and Cronar commercial polyesters are not transparent to wavelengths below 300 Ill/l. and thus prevent a good portion of the ultraviolet range from reaching the biimidazole.

It is, therefore, an object of the present invention to extend the spectral sensitivity of the ultraviolet light-absorbing hexaarylbiimidazoles to longer Wavelength visible light and thereby provide visible-light activated hexaarylbiimidazole compositions useful as light screens and photooxidants, as described in British Pat. 997,396; and to provide novel hexaarylbiimidazole/leuco dye imaging compositions responsive to visible as well as ultraviolet light, useful as described in British Pat. 1,047,569.

SUMMARY OF THE INVENTION This invention is directed to a visible-light sensitive composition comprising (A) a selected hexaarylbiimidazole which has its principal light absorption bands in the ultraviolet region of the electromagnetic radiation spectrum and is dissociable to triarylimidazolyl radicals on being irradiated with absorbable ultraviolet light, and (B) at least one hydroxyphthalein dye having its principal light absorption bands in the visible region of the spec trum, present in an amount sufficient to transfer energy to the hexaarylbiimidazole.

The invention is also directed to a composition containing components (A) and (B) above and (C) a leuco dye oxidizable to the colored form by triarylimidazolyl radicals.

The invention is further directed to a method for sensitizing a hexaarylbiimidazole to visible light which cornprises irradiating it with visible light in the presence of an energy transfer amount of a visible light-absorbing hydroxyphthalein dye, said visible light having wavelengths corresponding to the visible-light absorption bands of the hydroxyphthalein dye; and toe. method of imagewise exposing a biimidazole/hydroxyphthalein dye/leuco dye imaging composition as heretofore defined to a colorforming dosage of visible light.

The effect of the hydroxyphthalein dye seems catalytic since only small amounts are normally required. The actual quantity employed depends on the particular biimidazole and the effect desired, but generally ranges from about 0.001 to 0.1 mole per mole of the biimidazole (preferably .0l-0.05 mole/mole). Smaller amounts may be used, but do not always give the desired effect, while larger amounts are often unnecessary. When the leuco dye is present, it is normally employed in amounts ranging from 0.1 to 10 moles per mole of biimidazole (preferably 0.5 to 2 mole/mole).

While the sensitization mechanism is not known with certainty, it is considered that when compositions of this invention comprising the biimidazole and the hydroxyphthalein dye are irradiated with such relatively long wavelength light as visible light, the biimidazole is substantially unaffected and remains in its ground (unactivated) state. The hydroxyphthalein dye, however, responding directly, absorbs the light and is activated to at least one excited energy transfer state. In such state, the activated dye transfers absorbed energy to the biimidazole, for example through collision or resonance interaction, and returns to the ground state, thus becoming available again for activation. The thus-indirectly-activated biimidazole dissociates into imidazolyl radicals.

The overall sensitization process can be represented as visible light no activation visible light S Where S is the hydroxyphthalein sensitizer, S its ac tivated energy transfer state; LL is the biimidazole (dimer), LL* its activated, dissociable state; L- is the resulting imidazolyl radical.

The subsequent fate of the inherently colored and energy-rich imidazolyl radicals and their utilization in accordance with the various embodiments of this invention depends on the absence or presence of other substances that are reactive towards the radicals. Thus, in formulating light screens or windows containing the biimidazole/hydroxyphthalein sensitizer compositions of this invention, there will usually be employed other components, such as solvents, plasticizers and binders or carriers as described in said British Patent 997,396 that are substantially inert, i.e., resistant, to oxidation by the imidazolyl radicals. In this embodiment, the sensitization process manifests itself as a color change, attributable to formation of the inherently colored triarylimidazolyl radical (Equation 3 above). When the light source is removed, the color fades as the radicals dimerize (Equation 4).

The imidazolyl radicals are useful oxidants in the compositions of the invention involving the leuco dyes, as schematically illustrated in Equation where DH for example, is an oxidizable substance such as a leuco dye, D is the oxidation product (dye), and LH is the reduction product (triarylimidazole). Thus the imidazolyl radicals. In this embodiment, the sensitization larly useful as visible light actuated photooxidants for a variety of substrates, including leuco dyes, and the biimidazole/sensitizer/leuco dye compositions of this invention constitute the basic ingredient of visible light actuated imaging systems. This latter composition can also include other components such as solvents, plasticizers, binders or carriers as described in said British Pat. 1,047,796 that are substantially inert to composition ingredients.

DETAILED DESCRIPTION OF THE INVENTION (A) The hydroxyphthalein dye component The energy transfer hydroxyphthalein dyes utilized in the present invention are those hydroxyphthalein dye acids or salts thereof, e.g. alkali metal salts, that contain at least one, and normally two hydroxyl groups, and may contain one or more other substituents. Such dyes characteristically absorb visible light maximally in the 400600 m range. They also absorb in the near ultraviolet region, at wavelengths longer than absorption maxima of the biimidazoles, and are capable of transferring such relatively short-wavelength light as well as the longerwavelength visible light to the biimidazoles.

Halogenated hydroxyphthalein dyes are preferred and compounds where the halogens are chloro, bromo or iodo groups are readily available and are most preferred. Representative energy transfer hydroxyphthaleins are: fluorescein (Color Index 45350, Acid Yellow 73); 2,4',- 5',7-tetrachlorofluorescein (C.I. 45366); 4',5'-dibromofluorescein (C.I. 45370, including Eosin 8 HG); 2',4',5',- 7'-tetrabromofluorescein (CI. 45380, exemplified by Eosin Y); 4,7-dichloro-4',5 dibromofluorescein (CI. 45375, Phloxine N); 4,7-dichloro 2',4',5,7'-tetrabromofluorescein (CI. 45505 Phloxine); 4,5,6,7-tetrachloro-2',4,5',7- tetrabromofluorescein (CI. 45410); 4,5'-diiodofluorescein (CI. 45425, including Erythrosin 6G); 2',4',5',7'-tetraiodofiuorescein (CI. 45430, Erythrosin B); 4,7-dichloro- 2',4,5',7-tetraiodofluorescein (CI. 45435, Rose Bengal); and 4,567 tetrachloro-2',4,5,7 tetraiodofluorescein (C.I. 45440).

These dyes may be represented by the formula notation:

wherein M is hydrogen or a salt-forming cation, usually alkali metal, and normally sodium; X is hydrogen or halogen, normally chlorine, bromine or iodine (preferably it is halogen, particularly bromine or iodine as in erythrosin and eosin); Y is hydrogen or halogen, normally chlorine, bromine or iodine (preferably it is chlorine as in Rose bengal, erythrosin and eosin). The preferred sensitizers have from 2 to 4 X :Br or I and from 0 to 2 Y=Cl substituents.

(B) The hexaarylbiimidazole component This component is a 2,2,4,4',5,5-hexaarylbiimidazole which absorbs maximally in the ultraviolet light range and is dissociable by such light to the corresponding 2,4,5- triarylimidazolyl radical. They generally exhibit principal absorption bands in the 250275 I1'l,u region, although the absorption bands tend to tail out to include wavelengths as low as 220 m and as high as about 420; they thus normally require light rich in the 250275 m wavelengths for their dissociation. The aryl groups may contain 6-14 carbon atoms. The 4,4',5 and 5' aryl groups may each contain from 0 to 3 substituents that are free of a hydrogen capable of reacting with methyl magnesium halide, i.e., free of Zerewitenoif hydrogen. The 2 and 2' aryl groups must contain one substituent ortho to the carbon attached to the imidazolyl nucleus that is free of Zerewitenoff hydrogen and in addition has a parasigma value below 0.8. The 2 and 2 aryl groups can also contain from 0 to 3 substituents defined the same as the substituents on the 4,4',5 and 5' aryl groups. Lastly, no aryl group can contain more than one substituent ortho to the carbon atom attached to the imidazolyl nucleus. Preferably, the substituents on the aryl groups have sigma values in the range of .5 to +.8. Representative substi tuents and their sigma values (relative to H=.00), as given by Jaffe, Chem. Rev. 53, 219-233 (1953) are: methyl (0.17), ethyl (0.15), t-butyl (0-20), phenyl (0.01), nitro (0.78), trifluoromethyl (0.55), chloromethyl (0.18), cyanomethyl (0.01), 2-carboxyethyl (-0.07), butoxy (-0.32), phenoxy (0.03), fluoro- (0.06), chloro (0.23), bromo (0.23), iodo (0.28), methylthio (0.05), methylsulfonyl (0.73), carboxyl (0.27), ethoxycarbonyl (0.52), and cyano (0.63). Thus, the substituents may be halogen, cyano, hydrocarbyl (including alkyl, haloalkyl, cyanoalkyl, and aryl), alkoxyl, aroxyl, alkylthio, arylthio, alkyl sulfonyl, arylsulfonyl, and nitro. Preferably the aryl radicals are carbocyclic, particularly phenyl, and the substituents have Hammett sigma values in the range .4 to +.4, particularly lower alkyl, lower alkoxy, chlorine, fluorine and bromine groups.

In a preferred biimidazole class, the 2 and 2 aryl groups are phenyl rings bearing an ortho substituent having a Hammett sigma value in the range .4 to +.4 and in particular are lower alkyl, lower alkoxy, chlorine, fluorine or bromine groups.

Representative hexaarylbiimidazoles which mav be used in the practice of this invention include:

2,2'-bis o-bromophenyl) -4,4',5 ,5

tetraphenylbiimidazole 2,2'-bis o-chlorophenyl -4,4',5 ,5 '-tetrakis pmeth oxyphenyl -biimidazole 2,2'-bis o-chlorophenyl -4,4',5 ,5

tetraphenylbiimidazole 2,2'-bis 2,4-dichlorophenyl -4,4,5 ,5

tetraphenylbiimidazole 2,2'-bis 2,4-dimethoxyphenyl) -4,4',5 ,5

tetraphenylbiimidazole 2,2-bis o-ethoxyphenyl -4,4', 5,5

tetraphenylbiimidazole 2,2-bis (o-fiuorophenyl -4,4',5 ,5

tetraphenylbiimid azole 2,2-bis( o-hexoxyphenyl -4,4,5 ,5

tetraphenylbiimidazole 2,2'-bis( o-chlorophenyl) -4,4',5 ,5 -tetrakis (mmethoxyphenyl) biimidazole 2,2'-bis o-methoxyphenyl -4,4',5,5'-

tetraphenylbiimidazole 2,2'-di-o-tolyl-4,4',5 ,5 '-tetraphenylbiimidazole 2,2'-di-2,4-xylyl-4,4',5 ,5 '-tetraphenylbiimidazole 2,2'-bis (o-nitrophenyl) -4,4',5 ,5 '-tetraphenylbiimidazole Such biimidazoles are described more fully in said British Pat. 997,396.

(C) The leuco dye component The leuco dye component together with the biimidazole and the hydroxyphthalein forms one embodiment of this invention. By the term leuco dye" is meant the colorless (i.e., the reduced) form of a dye compound which upon oxidation becomes colored. When present in the composition of this invention, it is oxidized to its colored form by the imidazolyl radical.

Leuco dyes which may be oxidized to color by triarylimidazolyl radicals generated by this invention 1nclude: aminotriarylmethanes, aminoxanthenes, am nothioxanthenes, amino 9,10 dihydroacridines, aminophenoxazines, aminophenothiazines, aminodihydrophenazines, aaminodiphenylmethanes, leuco indarnines, aminohydrocinnamic acids (cyanoethanes, leuco methines), hydrazines, leuco indigoid dyes, amino-2,3-dihydroanthraquinones, tetrahalo-p,p'biphenols, 2(p hydroxyphenyD- 4,5-diphenylimidazoles, phenethylanilines, and the like.

The aminotriarylmethanes are preferred, especially those containing tertiary amino groups, and in particular those wherein (1) at least two aryls are phenyls having (a) a para-R R N substituent, where R and R are C -C alkyl, 2-hydroxyethyl, 2-cyanoethyl, benzyl or phenyl, and (b) an ortho C -C alkyl, C -C alkoxy, fiuoro, chloro, or bromo-substituent and (2) the third aryl, when different from the first two, is thienyl, furyl, phenyl or phenyl substituted with one or more C -C alkyl, C -C alkoxy, methylenedioxy, fluoro, chloro, bromo, dialkylamino, alkylthio or benzylthio groups.

Representative aminotriarlymethanes that may be used in this invention follow:

bis(2-bromo-4-diethylaminophenyl) phenylmethane,

bis 2-butoxy-4-diethylaminophenyl phenylmethane,

bis [4- 2,cyanoethyl) (2-hydroxyethyl amino-o-tolyl] (p-benzylthiophenyl methane,

bis [4-(2-cyanoethyl) (Z-hydroxyethyl) amino-o-tolyl1- Z-thienylmethane,

bis (4-dibutylamino-o-tolyl 2-thienylmethane,

bis (4-diethylamino-Z-ethylphenyl) (3 ,4-methylenedioxyphenyl methane,

bis (4-diethylamino-2-fluorophenyl) (p-benzylthiophenyl) methane,

bis (4-diethylamino-2-fluorophenyl) (3,4-methylenedixyphenyl methane,

bis(4-diethylamino-Z-methoxyphenyl) (p-nitrophenyl) methane,

bis (4-diethylaminol-naphthyl) (4-diethylamino-o-tolyl) methane,

bis(4-diethylamino-o-tolyl) (p-benzylthiophenyl) methane,

bis (4-diethylamino-o-tolyl) (2,4-dimethoxyphenyl) methane,

bis (4-diethylamino-o-tolyl Z-furylmethane,

bis (4-diethylarnino-o tolyl) (p methoxyphenyl methane,

bis (4-diethylamino-o-tolyl) 3,4methylenedioxyphenyl) methane,

bis (4-diethylamino-o-tolyl) (p-methylthiophenyl) methane,

bis (4-diethylamino-o-tolyl) l-naphthylmethane,

bis (4-diethylamino-o-tolyl) phenylmethane,

bis(4-diethylamino-o-tolyl Z-thienylmethane,

tris(4-dimethylamino-2-chlorophenyl) methane,

bis( 4-dimethylamino-2,5-dimethylphenyl phenylmethane.

bis (4-dimethylamino-Z-hexylphenyl) (p-butylthiophenyl) methane,

bis 4-dimethylamino-o-tolyl) (o-bromophenyl methane,

bis [4- (N-ethylanilino -o-tolyl] (3 ,4-dibutoxyphenyl) methane,

bis (4-ethylbenzylamino-o-tolyl) (p-methoxyphenyl) methane,

bis [4-bis (Z-hydroxyethyl amino-2-fiuorophenyl] (pbenzylthiophenyl methane,

tris (4-diethylamino-o-tolyl methane, and

tris (p-dioctylamino-o-tolyl methane.

These amino substituted cationic dye precursors are generally employed in the color forming process as salts of strong acids, including Lewis acids, such as hydrochloric, hydrobromic, sulfuric, nitric, phosphoric, acetic, oxalic, p-toluenesulfonic, zinc chloride, zinc bromide, and ferric chloride, the proportion of acid varying from 0.33 mole to one mole per amino group.

These leuco dyes are described more fully in said Britist Pat. 1,047,569.

As mentioned previously, the compositions of this invention can and usually do contain inert solvents, plasticizers, binders, substrates and the like (0.5% by weight or more of each based on the total composition weight).

The compositions of this invention are prepared simply by mixing them together, usually in a solvent and/or binder. The solution can then be applied to a substrate in usual fashion, e.g., spray, brush, roller, dipping, and the like.

Suitable solvents are those inert to the other ingredients of the composition and include amides such as N,N- dimethylformamide, N,N-dimethylacetamide; alcohols such as methanol, ethanol, l-propanol, 2-propanol, butanol, ethylene glycol; ketones such as acetone, methyl ethyl ketone, S-pentannone; halocarbons such as methylene chloride, chloroform, 1,1,2-trichloroethane, and 1,1, 2,2-tetrachloroethane; polyethylene glycols; esters, e.g., ethyl acetate and ethyl benzoate; aryls such as benzene, o-dichlorohenzene and toluene; dimethylsulfoxide, pyridine, acetonitrile, tetrahydrofuran, dioxane, 1,1,2-trichloroethylene, 1-methyl-2-oxohexamethyleneimine, and mixtures thereof.

Sutable inert plasticizers include the polyethyleneglycols, such as the commercially available car-bowaxes, and related materials, such as substituted henol-ethylene oxide adducts, for example the products obtained from p-phenylphenol and 6 moles ethylene oxide, and from p-nonylphenol and 2 moles ethylene oxide, including commercially available materials such as the Igepal alkyl phenoxy polyoxyethylene ethanols.

Polymeric materials, particularly light-transparent and film-forming polymers, are useful as inert binders, and carriers for the essential ingredients described above; thus, bi-imidazole, leuco dye, sensitizer and polymer, with or without a mutual solvent, may be mixed, then sprayed, extruded, cast, pressed or otherwise formed into supported or unsupported films or shaped articles. Representative polymers are polyvinyl alcohol, ethyl cellulose, polyvinyl chloride, polystyrene, polyvinyl acetate, poly(methyl methacrylate), cellulose acetate, cellulose butyrate, copolymers of vinyl monomers, gelatin, and polyethylene. Other suitable inert materials which may be used include glasses, resins, and waxes.

Typical inert substrates include materials commonly used in the graphic arts and in decorative applications, such as paper ranging from tissue paper to heavy cardboard; films of plastics and polymeric materials such as regenerated cellulose, cellulose acetate, cellulose nitrate, polyester of glycol and terephthalic acid, vinyl polymers and copolymers, polyethylene, polyvinylacetate, polymethyl and methacrylate, polyvinylchloride; textile fabrics; glass; wood and metals.

Any convenient light source providing wavelengths in the visible region of the spectrum that overlap the sensitizing dyes absorption bands may be used to activate the light-sensitive compositions for imidazolyl radical and image formation. The light may come from natural or artificial sources, may be monochromatic or polychromatic, incoherent or coherent, and for high efficiency should correspond closely in wavelengths to the principal absorption bands of the sensitizing dye employed and should be sufficiently intense to activate a substantial pro portion of the sensitizer. Also, it may often be advantageous to employ visible light in conjunction with ultraviolet light normally required to dissociate the dimer to increase the speed of imidazolyl radical and image formation.

(D) Examples Representative examples further illustrating the invention follow, and are not meant to be limiting:

EXAMPLES l-3 Visible-light sensitive compositions of this invention containing the following ingredients were prepared as so- For testing, each solution was mixed with 6.8% by weight of a polyethylene ether having an average molecular weight of about 550 as a binder, and spotted on 1" x 3" strips of Whatman No. 1 filter paper. After the acetone had evaporated, the papers were irradiated by contact flashing with a Xenon flash lamp (HiCo Lite, emitting ultraviolet and visible light above 200 m and approximating ordinary sun light), directly and through a series of sharp cut off filters, as designated below, which transmit wavelengths longer than that specified but substantially no shorter wavelengths.

Available as Cut 01f Corning wavefilter No./ length, Filter glass No. length, my

This test demonstrates biimidazole photolysis to triarylimidazolyl ardicals through reaction of the radicals with a triarylmethane leuco dye, evidenced by formation of the triarylmethane dyes characteristic blue color.

The more effective the system to generate triarlyirnidazolyl radicals under a given light stimulus, the deeper the color, with intense medium light faint blue.

Likewise prepared and tested were control solutions with biimidazole or sensitizer to show that both are needed and the sensitizer itself is not a photooxidant for the leuco dye.

The results are tabulated as follows, wherein the light intensity of the irradiated composition is given by the terms intense, medium, light, and faint and wherein the dashed lines denote that the composition was not tested at the given wave length.

The preceding results establish that (1) the biimidazole is the photooxidant and its activation in the absence of sensitizer requires relatively short wavelength light, (2) the sensitizer significantly extends the special sensitivity of the biimidazole towards longer wavelength light, permitting it to eifectively utilize near ultraviolet and visible light for activation.

The preceding representative examples may be varied within the scope of the present total specification disclosure, as understood and practiced by one skilled in the art, to achieve essentially the same results.

As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that this invention is not limited to the specific embodiments thereof.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A visible-light sensitive composition comprising (A) a 2,2,4,4',5,5'-hexaarylbiimidazole wherein each aryl group contains 6-14 carbon atoms and is substituted with from 0 to 3 substituents free of a hydrogen capable of reacting with methyl magnesium halide; with the proviso that each of the 2 and 2' aryl groups contains one substituent ortho to the carbon atom attached to the imidazolyl nucleus, said ortho substituent having a sigma value below 0.8 and being free of a hydrogen capable of reacting with methyl magnesium halide; and

(B) at least one hydroxyphthalein dye or salt thereof represented by the formula wherein M is hydrogen or an alkali metal cation; X is hydrogen or halogen; and Y is hydrogen or halogen, present in an amount sufiicient to dissociate the biimidazole to imidazolyl free radicals.

2. The composition of claim 1 which contains additionally (C) the leuco form of a dye selected from the class consisting of an aminotriarylmethane, an aminoxanthene, an aminothioxanthene, an amino-9,10-dihydroacridine, an aminophenoxazine, an aminophenothiazine, an aminodihydrophenazine, an aminodiphenylmethane, a leuco indamine, an aminohydrocinnamic acid, a hydrazine, a leuco indigoid dye, an amino-2,3-dihydroanthraquinone, a tetrahalo p,p'-biphenol, a 2(p-hydroxypheny1)-4,5-diphenyl-imidazole, and a phenethylaniline.

3. The composition of claim 1 wherein in component VISIBLE-LIGHT SENSITIZED BIIMIDAZOLE PHOTOLYSIS Filter/color formation Biimid- None/ azole Sensitizer 300 Inp 1/370 2/430 3/470 4/520 5/560 Example:

1 Medium Intense Medinm Light Faint. Yes Erythrosin do "do do ntense Medium Light. 3 Yes Rose Bengaluludo .d0 do Control:

No do Light. Faint None. None None. Eosin Y None Erythrosin B d0 Rose Benga1. .d No

(A) each aryl group is phenyl and any substituents thereon are lower alkyl, lower alkoxy, chlorine, fluorine or bromine, and wherein compound (B) is a halogenated hydroxyphthalein defined as in claim 1.

4. The composition of claim 3 wherein component (A) is 2,2 bis(o-chlorophenyl)-4,4',5,5-tetraphenylbiimidazole and component (B) is eosin, erythrosin or rose bengal.

5. The composition of claim 3 wherein component (A) is 2,2 bis(o-chlorophenyl)-4,4',5,5'-tetrakis(m-methoxyphenyDbiimidazole and component (B) is eosin, erythrosin or rose bengal.

6. The composition of claim 3 which contains additionally (C) the leuco form of the acid salt of an aminotriarylmethane dye. V

7. The composition of claim 6 wherein in component (A) the 2 and 2' phenyl groups are each substituted with a chlorine in the ortho position.

8. The composition of claim 7 wherein component (C) is the acid salt of an aminodiphenylarylmethane dye in which the two phenyl groups have (1) an R R N group in the para position wherein R and R are each C -C1 alkyl, Z-hydroxyethyl, 2cyanoethyl, benzyl or phenyl, and have (2) a substituent ortho to the carbon attached to the methane carbon, selected from lower alkyl, lower alkoxy, fluoro, chloro or bromo; and the aryl group is thienyl, furyl, phenyl or phenyl substituted with lower alkyl, lower alkoxy, methylenedioxy, fluoro, chloro, bromo, diethylamino, lower alkylthio or benzylthio.

9. The composition of claim 8 wherein in the aminotriarylmethane, the two phenyl groups having an R R N group are each p-diloweralkylamino-o-tolyl.

10. The composition of claim 4 which contains additionally C) tris (N,N-diethylamino-o-tolyl)methane.

11. The composition of claim 5 which contains additionally (C) tris(N,N-diethylamino-o-tolyl)methane.

12. A coating composition comprising a composition of claim 6 in an inert solvent.

13. A coated composition comprising an inert substrate coated with a composition of claim 6.

14. The coated composition of claim 13 wherein the substrate is paper.

15. The coated composition of claim 13 wherein the substrate is a film.

16. Process which comprises irradiating a composition of claim 1 with visible light in a color-forming dosage.

17. Process which comprises irradiating a composition of claim 2 with visible light in a color-forming dosage.

18. Process which comprises irradiating a composition of claim 13 with visible light in a color-forming dosage.

References Cited UNITED STATES PATENTS 7/1963 Levinos 96-36.3X 7/1968 MacLachlan 96-90 US. Cl. X.R. 96-88; 252-300 UNITED STATES PATENT OFFICE CERTIFICATE ()F CORRECTION Patent No. 2 3,75 Dated February 1 19?].

Inventor(s) PETER WALKER It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

I In Column 3, line 9, "imidazolyl radicals. In this embodiment, the sensitization" should be deleted, and biimidazole/hydroxyphthalein combination are particu inserted in Claim 1, the formula set forth in part (B) shouI be deleted and the following formula substituted therefor:

Signed and sealed this 21st day of September 1971 Si ned and sealed this 21st day of September 1971.

Attasting Officer Acting Commissioner of Pate 

